1. Field of the Invention
The present invention relates to a final reduction gear device, and more particularly to a final reduction gear device comprising an orthogonal axis gear, and a speed reduction mechanism portion employing a planetary gear.
2. Description of the Related Art
In an orthogonal axis gear of a conventional final reduction gear device, as shown in FIG. 3, a skew gear, or in other words a so-called hypoid gear, is used (see Japanese Unexamined Patent Application Publication 2007-40339, for example), but as the offset of the hypoid gear increases, friction during a low load period increases. Therefore, as shown in FIG. 4, the efficiency of the final reduction gear device may be improved by using a zero offset intersecting axis gear, or in other words a so-called bevel gear or spiral bevel gear (1), in place of the hypoid gear and interposing a speed reduction mechanism portion (3) employing a planetary gear between the bevel gear (1) and a differential gear mechanism portion (2) (see Japanese Unexamined Patent Application Publication S64-6548, for example).
However, in the final reduction gear device of Japanese Unexamined Patent Application Publication S64-6548, as shown in FIG. 4, a housing (6) that connects a bevel ring gear (4) to a sun gear (5) is supported while straddling left and right output shafts (8), (8) via a differential box (7), and as a result, the size and weight of the final reduction gear device increase.
To solve this problem, the housing (6) may be formed such that it is supported on one of the left and right output shafts (8), (8), as shown in FIG. 5, thereby increasing the compactness of the final reduction gear device. However, although the compactness of the final reduction gear device is increased, the bevel ring gear (4) and an internal gear (9) of the speed reduction mechanism portion (3) are supported in a cantilevered fashion, and therefore, unless the support rigidity of the bevel ring gear (4) and the internal gear (9) is improved, the rotation of the bevel ring gear (4) becomes unstable during power transmission, leading to tooth bearing defects, defects in the alignment of a planetary gear (10) of the speed reduction mechanism portion (3) and so on, a degradation in efficiency, and the generation of noise.